abump: fix set -e issue

It appears that when the subshell has a ||, the 'set -e' within
subshell gets invalidated.

This will work as expected:

 ( set -e; false; echo "should not get here" )

While this will not work as expected:

 ( set -e; false; echo "should not get here" ) || false

We resolve it by using $? to detect the status of subshell. We also let
the exitcode indicate how many packages that failed.

While here we also refactor it so most of the loop happens within the
subshell. This lets us set (or increase) rc variable once, and it
reduces number of forks which gives slightly better performance.